52 by Bjørn T. Larsen1, Snorre Olaussen2, Bjørn Sundvoll3, and Michel Heeremans4 The Permo-Carboniferous Oslo Rift through six stages and 65 million years 1 Det Norske Oljeselskp ASA, Norway. E-mail: [email protected] 2 Eni Norge AS. E-mail: [email protected] 3 NHM, UiO. E-mail: [email protected] 4 Inst. for Geofag, UiO. E-mail: [email protected] The Oslo Rift is the northernmost part of the Rotliegen- des basin system in Europe. The rift was formed by lithospheric stretching north of the Tornquist fault sys- tem and is related tectonically and in time to the last phase of the Variscan orogeny. The main graben form- ing period in the Oslo Region began in Late Carbonif- erous, culminating some 20–30 Ma later with extensive volcanism and rifting, and later with uplift and emplacement of major batholiths. It ended with a final termination of intrusions in the Early Triassic, some 65 Ma after the tectonic and magmatic onset. We divide the geological development of the rift into six stages. Sediments, even with marine incursions occur exclusively during the forerunner to rifting. The mag- matic products in the Oslo Rift vary in composition and are unevenly distributed through the six stages along the length of the structure. Introduction The Oslo Palaeorift (Figure 1) contributed to the onset of a pro- longed period of extensional faulting and volcanism in NW Europe, which lasted throughout the Late Palaeozoic and the Mesozoic eras. Widespread rifting and magmatism developed north of the foreland of the Variscan Orogen during the latest Carboniferous and contin- ued in some of the areas, like the Oslo Rift, all through the Permian period. We review the geological development of the Oslo Rift through its six stages of development (Ramberg and Larsen, 1978, Sundvoll et al., 1990, Olaussen et al., 1994), focusing on the four first—their lavas, sediments and tectonic structure, and briefly put it into the plate tectonic framework of NW Europe. The Variscan orogeny, the Tornquist line and the Oslo Rift Figure 1 Simplified geological map of the Oslo Graben area. The Oslo Rift sediments exhibit great similarities to the Lower Brown—includes both volcanics, sediments and large dykes related Rotliegendes in the Northern European Permian Basin and in Katte- to the Oslo Graben; Carboniferous-Permian age. Red—large gat and may be regarded as a prolonged northern arm of the North- Permian batholithic intrusions. Small blue dots—Permian gabbroic ern Permian Basin. The Skagerrak Graben is the southern part of the intrusions. Green—Lower Palaeozoic sediments. Yellow—the Oslo Rift and is the link between the two tectonic systems (Heere- Caledonian thrust front. White—Pre-Cambrian basement rocks. mans et al., 2004). Abbreviations for different areas: Brum. = Brumunddal, Krok. = Recent reviews of post-Variscan tectonics in Western Europe Krokskogen, and for the caldera volcanoes; Øy = Øyangen, He = (McCann et al., 2006; Ziegler et al., 2006) have described the genetic Heggelia, Ni = Nittedal, Bæ = Bærum, Gl = Glitrevann, Dr = relations and the timing between the Variscan orogeny and subse- Drammen, Sa = Sande, Hi = Hillestad and Ra = Ramnes. March 2008 53 Figure 2 Simplified tectonic overview of West Europe with the Variscan front, the Tornquist fault system and the Oslo Rift. Also shown are the pre-rift configurations with the Caledonian structures and the boundary of the Fennoscandian Craton. quent large, mostly NW-SE striking, wrench fault systems. The largest and northernmost is the Sorgenfrei-Tornquist Zone (Figure 2) that strikes across Scania (Skåne) into the North Sea (north of the Ringkøping-Fyn High), developing at least partly as a dextral strike- slip fault system. North of this fault, extensional stress fields devel- Figure 3 The graben segments and the graben polarity, the oped widespread rifting, being linked to the late stages of the master faults, the accommodation structures and the transfer fault orogeny and to the strike-slip faulting (Heeremans et al., 1997). Rifts in the Oslo Rift. Abbreviations of the structural nomenclature: formed both inside the orogen and in the foreland to the north, even R.F. = Rendalen fault, S.H. = Solberg Horst, R.H.F = extending into the Fennoscandian Craton. The northernmost and the Randsfjorden-Hunnselv Fault, K.K.T.F. = Krokkleiva- largest of these structures was the Oslo Rift. Kjaglidalen Transfer Fault, E.T.F. = Ekeberg. Transfer Fault. Warr (2000) divided the development of the Variscan orogenic O.F. = Oslofjord Fault, and L.A.Z. = Langesund Accommodation system in NW Europe into four phases, separated both in time and in Zone. Li = Lillehammer, H = Hamar, D = Drammen, K = different areas. The last of the four phases is named the Asturian Kongsberg, M = Moss, S = Skien, La = Larvik. phase and is generally Westphalian to Early Permian in age. Both Ziegler et al. (2006) and McCann et al. (2006) described it as the Finally, the offshore Skagerrak Graben represents the southern- consolidation phase of the Variscan Fold belt and gave an age most part of the Oslo Rift, and abuts towards the NW-SE trending 305Ma as the critical decline of the Variscan orogeny and the onset Sorgenfrei-Tornquist Zone in the south. The two Akershus and Vest- of rifting. Latest Carboniferous to earliest Permian was the time for fold graben segments form the classical Oslo Graben which is the onset of the Oslo Rift, leading up to its climax of both tectonic 220 km long and about 60 km wide. Adding the 100 km long Ren- and magmatic activity (Sundvoll et al., 1990; Heeremans et al., dalen Graben in the north, and the 180 km offshore Skagerrak 1997). Graben in the south makes the total length of Oslo Rift about 500 km. The Skagerrak Graben is broader than the other segments to the north, and is composed of several more or less overlapping The Oslo Rift architecture and grabens (Heeremans et al., 2004). The rift axis here strikes NE-SW, perpendicular to the Sorgenfrei-Tornquist fault system. nomenclature The architecture of the Oslo Rift is very much like that of other well The petrogenesis of a high volcanicity rift known rift structures. Most have polarity off-set of grabens along the length of the rift axis, as described e.g. by Rosendahl (1987). The Larsen and Sundvoll (1984) summarized the Oslo Graben part of the Oslo Graben (Figure 3) was subdivided into two rift segments with Oslo Rift as a north-south trending Permo-Carboniferous high-vol- opposite subsidence polarity (Ramberg and Larsen, 1978). The canicity continental rift system, much like the recent East African Akershus Graben segment has an E-verging master fault (the Rands- rifts of Kenya and Ethiopia. The high volume of volcanics filling the fjord-Hunnselv Fault) to the north, while the Vestfold Graben seg- rift is a feature common to both, and distinguishes them from other ment has a W-verging master fault (the Oslofjord Fault) to the south. continental low-volcanicity rifts such as the Baikal Rift and the These two half grabens have their accommodation zone around the Viking Graben. These two categories of rifts are useful descriptive city of Oslo, with a joining fault to the west of Oslo in the Kjagli- end-members (Barberi et al., 1982). dalen-Krokkleiva Transfer Fault (Heeremans et al., 1997). Today, A thorough analysis of the available data from the Oslo Rift we add the third Rendalen Graben segment to the north of the Aker- was undertaken by Neumann et al. (2004). They discussed the shus Graben, also with a west-verging master fault system, the Ren- magma origin and concluded that at least three mantle components dalen Fault (Skjeseth, 1963; Larsen et al., 2006). The accommoda- have contributed to the petrogenesis of the basaltic magmas, the old- tion, or transfer system, between the Akershus Graben and the Ren- est apparently being derived from an enriched mantle source. This dalen Graben is represented by the NE-SW trending Solberg Horst, source was most likely located in the lithospheric mantle and might beside lake Mjøsa. have been metasomatically altered by older carbonatitic fluid-rich Episodes, Vol. 31, No. 1 54 melts (Anthony et al., 1989). The main mantle source for the basaltic magmatism was a prevalent depleted mantle. It may represent the composition of the base of the local lithospheric mantle, and the asthenosphere beneath, which partly melted in response to localized thinning of the lithosphere due to the extension. Anthony et al. (1989) also suggested an alternative scenario involving a mantle plume, with depleted characteris- tics, actively up-welling beneath the lithosphere. The most primitive lavas appear to involve low degree partial melting of one or more sublithospheric mantle sources. The rising man- tle-derived magmas were modified by shallow-level processes, including magmatic differentiation, general fractional crys- tallisation, magma mixing and lithospheric contamination that masked the geochemical signature of the mantle source. Large volumes of mantle-derived basaltic magma formed chambers near the Moho at c. 36 km depth. This also led to anatectic melting in the Precambrian host-rocks. Initial Sr isotopic ratios significantly above 0.7039 are typical of the syenitic and granitic rocks and imply influence of crustal contamination in the lower crust (Sundvoll et al., 1990). After 280 Ma, the rocks show a clear trend of increasing ini- tial ratios; mantle signature is only present in the larvikites and the rhomb-porphyry and basalt lavas. Sundvoll et al. (1990) interpreted the Sr-initial ratios to reflect the relative importance of mantle- versus crustal-derived melts. At c. 280 and 275 Ma, the magmatism became dominated by melts (syenitic and granitic) containing a larger crustal component. The mantle source had slowly become inactive, but mantle- derived magmas were still undergoing fractional crystallisa- tion in magma chambers in the lower crust giving rise to evolved rocks such as larvikites and late rhomb porphyry lavas, and to basaltic central volcanoes with shallower crustal magma chambers at c.